The present invention relates to magnetic resonance imaging systems and, in particular, to the radio frequency coils used in such systems.
Magnetic resonance imaging (MRI) utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field from the main magnet system (named B0xe2x80x94the main magnetic field in MRI physics). The magnetically polarized nuclear spins generate i; magnetic moments in the human body. The magnetic moments point in the direction of the main magnetic field in a steady state, and produce no useful information if they are not disturbed by any excitation.
The generation of the nuclear magnetic resonance (NMR) signal for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform radio frequency (RF) magnetic field (named B1 field or the excitation field). The B1 field is produced in the imaging region of interest by an RF transmit coil which is driven by a computer-controlled RF transmitter with a power amplifier. During excitation, the nuclear spin system absorbs magnetic energy, and the magnetic moments precess around the direction of the main magnetic field. After excitation, the precessing magnetic moments will go through a process of free induction decay (FID), releasing their absorbed energy and returning to the steady state. During the FID, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the human body. The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the human tissue. The receive RF coil can be either the transmit coil itself, or an independent receive-only RF coil. The NMR signal is used for producing MR images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system, which generate magnetic fields in the same direction of the main magnetic field, varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In a standard MRI system, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Since a high SNR ratio is the most desirable in MRI, special-purpose coils are used for RF reception to enhance the SNR from the volume of interest.
In practice, a well-designed specialty RF coil should have the following functional properties: high SNR, good uniformity, high unloaded quality factor (Q) of the resonance circuit, and high ratio of the unloaded to loaded Q factors. In addition, the coil device must be mechanically designed to facilitate patient handling and comfort, and to provide a protective barrier between the patient and the RF electronics. Another way to increase the SNR is by quadrature reception. In this method, NMR signals are detected in two orthogonal directions, which are in the transverse plane or perpendicular to the main magnetic field. The two signals are detected by two independent individual coils that cover the same volume of interest. With quadrature reception, the SNR can be increased by a factor of up to the square root of 2 over that of the individual linear coils.
Most of currently available knee coils are designed to image the knee only and foot/ankle coils (e.g., U.S. Pat. No. 5,361,764) to image the foot and ankle only.
U.S. Pat. No. 5,277,183, which is incorporated herein by reference, shows a coil that performs all three functions, but the coil design makes compromises between clinical versatility and imaging performance. This coil modifies the shape of a standard birdcage coil (U.S. Pat. No. 4,680,548) for receiving the toes of the foot when the foot is placed in the knee coil. The appendant volume for receiving the toes is shaped like a chimney, and therefore the coil has a nickname of xe2x80x9cchimney coilxe2x80x9d. One major drawback is that the chimney coil design deviates from the optimized birdcage structure, and therefore the RF current pattern is not optimized for the best image uniformity and SNR. The second major disadvantage is that the chimney coil does not work in MRI systems with have a vertical main magnetic field. Another drawback of the chimney coil is that it does not offer enough coverage for imaging the foot and ankle without sacrificing the image quality of the knee.
A MRI RF coil for use on a human leg having a knee, ankle and foot. The coil includes a knee coil section and a foot/ankle coil section. The sections are configurable into a boot-like structure.